Gas separation membrane with partial surfactant coating

ABSTRACT

A gas separation module includes fibers formed of a membrane which exhibits selectivity between gaseous components to be separated. The fibers are partially coated with a solution which enhances their selectivity, the fibers being uncoated in the vicinity of their feed ends. The partially coated fibers provide a good compromise between the goals of improved selectivity and enhanced product flow. The gaseous component that permeates through the membrane is made to flow in a direction opposite that of the main gas feed, due to a baffle that directs the permeate in the desired direction. The invention also includes a method and apparatus for making the partially coated module.

CROSS-REFERENCE TO PRIOR APPLICATION

This is a division of U.S. patent application Ser. No. 11/137,827, filedMay 25, 2005.

BACKGROUND OF THE INVENTION

The present invention relates to the field of non-cryogenic separationof gases into components, and provides a partially coated membrane whichenhances the efficiency of the gas separation process.

It has been known to use a polymeric membrane to separate air intocomponents. Various polymers have the property that they allow differentgases to flow through, or permeate, the membrane, at different rates. Apolymer used in air separation, for example, will pass oxygen andnitrogen at different rates. The gas that preferentially flows throughthe membrane wall is called the “permeate” gas, and the gas that tendsnot to flow through the membrane is called the “non-permeate” gas. Theselectivity of the membrane is a measure of the degree to which themembrane allows one component, but not the other, to pass through.

A membrane-based gas separation system has the inherent advantage thatthe system does not require the transportation, storage, and handling ofcryogenic liquids. Also, a membrane system requires relatively littleenergy. The membrane itself has no moving parts; the only moving part inthe overall membrane system is usually the compressor which provides thegas to be fed to the membrane.

A gas separation membrane unit is typically provided in the form of amodule containing a large number of small, hollow fibers made of theselected polymeric membrane material. The module is generallycylindrical, and terminates in a pair of tubesheets which anchor thehollow fibers. The tubesheets are impervious to gas. The fibers aremounted so as to extend through the tubesheets, so that gas flowingthrough the interior of the fibers (known in the art as the bore side)can effectively bypass the tubesheets. But gas flowing in the regionexternal to the fibers (known as the shell side) cannot pass through thetubesheets.

In operation, a gas is introduced into a membrane module, the gas beingdirected to flow through the bore side of the fibers. One component ofthe gas permeates through the fiber walls, and emerges on the shell sideof the fibers, while the other, non-permeate, component tends to flowstraight through the bores of the fibers. The non-permeate componentcomprises a product stream that emerges from the bore sides of thefibers at the outlet end of the module.

The effectiveness of a membrane in gas separation depends not only onthe inherent selectivity of the membrane, but also on its capability ofhandling a sufficiently large product flow. Gas permeates through themembrane due to the pressure differential between one side of themembrane and the other. Thus, to maintain the pressure differential, itis advantageous to remove the permeate gas from the vicinity of thefibers, after such gas has emerged on the shell side. Removal of thepermeate gas maximizes the partial pressure difference across themembrane, with respect to the permeate gas, along the length of themodule, thus improving both the productivity and recovery of the module.In the membrane module of the present invention, the permeate gas ismade to flow out of the module in a direction opposite to that of thebasic feed stream.

It has been known that certain materials, when coated onto a polymericmembrane, can enhance the selectivity of the membrane with regard tospecific types of gases to be separated. U.S. Pat. No. 5,141,530, thedisclosure of which is incorporated by reference herein, describes asolution, in water, of a non-ionic surfactant which, when applied to themembrane, improves its selectivity for the separation of air into oxygenand nitrogen, and thereby increases the efficiency of the air separationprocess. However, coating the membrane reduces the permeability of themembrane, so that while the coated membrane becomes more selective, italso processes less gas than an uncoated membrane.

In the above-cited patent, the entire lengths of the membrane fibers inthe module are coated, so as to provide more selectivity. But althoughthe product recovery is improved, the product flow rate is reduced, thusimpairing the utility of the system. If one used, instead, an uncoatedmodule, the product flow would be improved, but the product recoverywould be reduced. Also, an uncoated module will produce a higherpressure drop, along the length of the fiber, as compared with a coatedmodule.

The present invention provides a gas separation membrane module whichmaximizes the product recovery of the module, but which minimizes thepressure drop along the length of the module, for a given product flow.The module of the present invention therefore enjoys advantages of botha coated and an uncoated module, and combines the best features of both.The invention also includes a method of making the improved module.

SUMMARY OF THE INVENTION

The gas separation module of the present invention comprises a pluralityof hollow fibers, extending from a first tubesheet at the feed end ofthe module, to a second tubesheet at the product end of the module. Thefibers are formed of a membrane which exhibits selectivity betweengaseous components to be separated from a feed stream. The fibers arepartially coated with a solution which improves the selectivity of themembrane, the fibers being uncoated in the vicinity of the feed end. Inone preferred embodiment, the fibers may be coated along approximatelythree-quarters of their length.

In the module of the present invention, the permeate gas, whichpermeates through the walls of the fibers, flows out of the module in adirection opposite to that of the main gas feed, due to the effect of abaffle which allows the permeate gas to exit the module only in thevicinity of the feed end.

In one embodiment, the baffle extends from one tubesheet to the other,and the permeate gas escapes through openings formed in the baffle nearthe feed end. In another embodiment, the baffle does not extend all theway to the first tubesheet, leaving a gap between the first tubesheetand the baffle. The permeate gas escapes through this gap.

The invention also includes an apparatus for making the partially coatedmodule. The apparatus includes a module as described above, and areservoir, the module and the reservoir being linked by a valvedconduit. The module is mounted vertically with its product end facingdown, and the reservoir is filled with a solution which, when coatedonto the fibers of the module, enhances their selectivity. When thesolution is allowed to flow freely, by gravity, between the reservoirand the module, the liquid levels in the reservoir and the module becomeequal. The vertical position of the reservoir is adjusted so as toadjust the degree to which the fibers in the module are submerged. In apreferred embodiment, the fibers are coated along about three-quartersof their length, meaning that about one-quarter of the length of thefibers remains above the surface of the liquid in the module.

The module is connected to a drain valve for draining the solution fromthe module when the coating operation is complete, and a conduit fordirecting dry gas through the module, to dry the fibers after coating.

The present invention also includes the method of partially coating themodule using the apparatus described above, the coating being performedsuch that a portion of the fibers of the module remain uncoated near thefeed end.

The invention therefore has the primary object of providing amembrane-based gas separation module having improved performance.

The invention has the further object of providing a gas separationmodule having improved selectivity and improved product flow, whileminimizing pressure drop, for a given level of product purity.

The invention has the further object of improving the efficiency ofmembrane-based gas separation systems.

The invention has the further object of providing an apparatus andmethod for improving the selectivity of a membrane-based gas separationmodule.

The reader skilled in the art will recognize other objects andadvantages of the present invention, from a reading of the followingbrief description of the drawings, the detailed description of theinvention, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic diagram of an apparatus used to coat a gasseparation membrane module according to the present invention.

FIG. 2 provides a schematic diagram, showing the apparatus of FIG. 1,and also showing means for drying the module after coating.

FIG. 3 provides a cross-sectional view, partially schematic in nature,showing a gas separation membrane module made according to oneembodiment of the present invention.

FIG. 4 provides a view similar to that of FIG. 3, showing anotherembodiment of the gas separation membrane module of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a gas separation module defining amembrane formed as a plurality of hollow fibers, the fibers beingpartially coated with a material that improves the selectivity of themembrane.

FIG. 1 illustrates an apparatus used for performing the partial coating.Reservoir 1 holds solution 2, the solution including a non-ionicsurfactant of the type described in U.S. Pat. No. 5,141,530, or anyother solution which, when applied to a membrane, improves theselectivity of that membrane with respect to the gaseous componentsbeing separated.

Valve 3, when opened, allows solution 2 to flow by gravity, throughconduit 5, into membrane module 4. Module 4, which will be described inmore detail below, contains a plurality of hollow fibers, extending fromthe feed end 9 to the product end 10, the fibers being made of amaterial that exhibits selectivity with respect to the gaseouscomponents to be separated. The feed end is the end at which gas to beseparated is introduced into the module. The product end is the end fromwhich a product gas is withdrawn. Thus, when the module is used, gasflows from the feed end to the product end.

The module 4 has tubesheets, at the feed end and the product end, forholding the fibers. The tubesheets will be described in more detailbelow. The tubesheets both include at least one opening. Opening 21 atthe product end allows liquid to flow from reservoir 1. Opening 23 atthe feed end provides a vent which allows the liquid levels to becomeequal. As shown in FIG. 1, the module is oriented vertically, with theproduct end down.

The degree to which the fibers in the module are coated is determined bythe vertical position of the reservoir 1. As indicated by arrow 6, thereservoir is vertically movable, the arrow 6 being intended to representany means for moving the reservoir up or down. When the valve 3 isopened, and the solution flows into module 4, the liquid level in thereservoir 1 and the liquid level in the module 4 quickly become thesame. Thus, the level of liquid in the module 4 can be effectivelyadjusted, upward or downward, simply by moving the reservoir up or down.

According to the present invention, the portion of the module near thefeed end is left uncoated, while the portion near the product end iscoated. As used in this specification, the term “coating a module” meanscoating the fibers of the module. The degree to which the fibers in themodule are coated is clearly determined by the level of the solution inthe module. Thus, one adjusts the vertical position of the reservoir 1,thereby varying the percentage of the fibers in the module that will becoated. The percentage chosen depends on the performance desired fromthe module.

If there is no coating at all, the module is likely to have a somewhathigher product flow than a coated module, but with a reduced productrecovery (often defined as the ratio of product flow to the requiredfeed flow for a given product purity), as well as a higher pressuredrop. If the fiber is coated completely, the result will be higherproduct recovery, but with a lower product flow. It has been found thatcoating about three-quarters of the fiber length will provide a goodcompromise between the above considerations.

After the level of the solution has been adjusted as desired, the liquidlevel in the module should be maintained for a period of time sufficientto insure complete contact between the solution and the fibers, and toinsure uniformity of coating from one fiber to the next. The duration ofexposure to the solution may be the same as described in U.S. Pat. No.5,141,530. When the module has been exposed for a sufficient time, thesolution is drained from the module, and dry gas (usually air) is usedto dry the module.

To drain the solution from the module, one closes valve 3, and opensvalve 11. Thus, the solution drains through the valve 11, and no moresolution can flow out of the reservoir 1. The dry gas is delivered tothe module through conduit 8. The dry gas is passed from the feed end tothe product end of the module, so as to avoid the possibility ofunintentionally depositing any residual solution on the feed end of thefibers. The dry gas is made to flow through the system until the moduleis dry.

FIGS. 3 and 4 illustrate, in schematic form, two embodiments of themodule made according to the present invention. In both of thesefigures, the actual fibers are represented symbolically by horizontallines such as those indicated by numerals 41 and 51, it being understoodthat there are many more fibers than can be conveniently illustrated,and that the fibers extend from one tubesheet to the other.

In FIG. 3, the module includes tubesheet 30, located at the feed end,and tubesheet 32 located at the product end. The stippled region 34represents the region in which the fibers are coated, and thenon-stippled region 36 designates the region in which the fibers are notcoated. The arrows pointing from left to right, such as arrows 38,indicate the main flow of gas, through the bores of the fibers, from thefeed end to the product end. The arrows pointing from right to left,such as arrows 40, indicate the countercurrent flow of permeate gas thathas passed through the walls of the fiber. Thus, the gas flowing in thedirection indicated by arrows 40 flows on the shell side of the fibers.

The module of FIG. 3 also includes baffle 42, which encloses the module.Due to the fact that the baffle 42 and the tubesheet 32 aresubstantially impervious to gas, and due to the fact that the baffleabuts the tubesheet 32, the permeate gas on the shell side of the fiberscan flow only towards the left, i.e. back towards the feed end, wherethe gas exits through openings 44 in the baffle, as indicated by arrows46. The gas flows out because the feed gas is injected under pressure,and the permeate gas which emerges on the shell side of the fibers hasnowhere to go but through the openings, due to the barriers defined bythe baffle and the tubesheets. The only gas that penetrates thetubesheets is gas flowing inside the hollow fibers which themselves aremounted to, and pass through, the tubesheets. Arrow 48 represents theflow of product gas which has passed through the bores of the fibers andthrough the tubesheets as described above. The baffle therefore servesas a means for causing countercurrent flow of the permeate gas.

The module represented in FIG. 4 is similar to that of FIG. 3, exceptthat baffle 50 does not extend from one tubesheet to the other. Instead,the baffle defines gap 52, adjacent to tubesheet 54 at the feed end ofthe module. Permeate gas on the shell side of the fibers therefore exitsthrough this gap, as indicated by arrows 56.

In FIG. 4, the baffle is approximately coextensive with the coatedregion. However, the baffle could be longer or shorter than the coatedregion, and there is no inherent correlation between the length of thebaffle and the length of the coated region. The present invention isintended to include all of the above possibilities.

The modules made as described above have improved selectivity andimproved throughput, relative to gas separation modules of the priorart. The modules of the present invention also minimize the pressuredrop along the length of the module, for a given product flow.

In particular, it has been found that the modules of the presentinvention provide about 10-25% more product flow than modules of theprior art, while still improving product recovery by about 10%, andwhile minimizing the pressure drop along the length of the modules.

EXAMPLE

A module, formed of hollow fibers made of tetra-bromo-bis-apolycarbonate (TBBA-PC) was prepared and coated, in varying amounts,with a non-ionic surfactant water-based solution, namely Triton-X 100,having a concentration of 150 ppm. Triton-X is one of the materials usedas a coating in U.S. Pat. No. 5,141,530. The initial pressure of the gasinjected into the module was 135 psig, at a temperature of 25° C. Themodule was tested in the uncoated state, and with coatings of 50% and75% of the fiber length. To coat the fibers, the module was heldpartially submerged in the solution for four hours. The solution wasthen drained, and the module was dried for 16 hours with dry air. Foreach test, the module was operated to produce nitrogen at a purity of99%, and the following results were obtained for this level of purity.

The following table summarizes the results: Coating Product Flow ProductRecovery Pressure Drop (%) (scfh) (%) (psi) 0 2042 30.7 5.7 50 2005 31.94.3 75 1812 33.9 3.7

As shown by the table, the module that was 50% coated exhibited aproduct flow that was about 2% less, and a relative recovery that wasabout 4% more, with a pressure drop that was about 25% smaller, ascompared with the uncoated module. For the module that was 75% coated,the product flow was reduced by about 11%, and the relative recovery wasincreased by about 10%, with a pressure drop that was about 35% less, ascompared with the uncoated module.

In contrast, in U.S. Pat. No. 5,141,530, the fully-coated modulesdescribed in the patent showed reductions of 22-58% in product flow, andrelative recovery increases of 5-8%.

The pressure drops typically vary linearly with the product flow, butsince a significant amount of the feed flow is permeated near the feedend, in the uncoated part of the fiber, the present invention exhibits apressure drop decrease of 35%, with a corresponding drop in product flowof only 11%.

The membrane used to make the module of the present invention exhibitsselectivity, relative to the gases desired to be separated, even beforeit is coated. In particular, the selectivity of the uncoated membranemay be about 80% of the selectivity of the coated membrane. Also, thepermeability of the coated membrane may be about 50-60% of thepermeability of the uncoated membrane. Thus, the present inventioncomprises partially coating a membrane that is already selective for thegaseous components to be separated.

The invention can be modified in various ways. The invention is notlimited to a specific material for the membrane, the choice of materialbeing dictated by the application. For example, the principle of theinvention could be applied to the separation of gas into components, orto the separation of water vapor from air. The invention could also bepracticed with various coatings that improve the selectivity of themembrane. These and other modifications, which will be apparent to thereader skilled in the art, should be considered within the spirit andscope of the following claims.

1. Apparatus for enhancing performance of a gas separation module, themodule having a feed end and a product end, the module including fibersmade of a gas separation membrane, the fibers extending from the feedend to the product end, the apparatus comprising: a) a reservoir forstoring a solution which enhances selectivity of the gas separationmembrane, the reservoir having an outlet connected to a valve, b) a gasseparation module having a plurality of fibers extending from a feed endto a product end, the module being in fluid communication with thereservoir, the module being mounted with the product end oriented downand the feed end oriented up, and c) means for vertically adjusting aposition of the reservoir, so as to adjust a liquid level in the module,wherein the fibers of the module are partially coated with saidsolution, and wherein the fibers remain uncoated in a vicinity of thefeed end.
 2. The apparatus of claim 1, further comprising a drain valve,connected to the module, for draining the solution from the module whena coating operation is complete.
 3. The apparatus of claim 2, whereinthe apparatus further comprises a conduit for directing a drying gastowards the module, the conduit being connected to the feed end of themodule.
 4. A method of enhancing selectivity of a membrane-based gasseparation module, the module having a feed end and a product end, themodule including fibers extending from the feed end to the product end,the fibers being made of a membrane exhibiting selectivity betweengaseous components to be separated, the module including means fordirecting a permeate gas in a direction opposite that of a main feedstream, the method comprising: a) mounting the module vertically withthe product end facing down, b) partially filling the module with asolution which enhances selectivity of the membrane, the filling beingaccomplished by opening a valve connecting a reservoir with the module,wherein levels of solution in the module and the reservoir become equal,and wherein portions of the fibers of the module, in a vicinity of thefeed end, are not submerged in the solution, and c) adjusting a verticalposition of the reservoir so as to adjust a proportion of coating of thefibers of the module.
 5. The method of claim 4, further comprisingdraining the solution from the module, and drying the module by passinga dry gas through the module.